Abstract: Objective : To explore the inhibition of telomerase activity and malignant phenotype of the Dominant Negative human telomerase reverse transcriptase gene (DN-hTERT) transfected into MCF7 breast cancer cells. Methods : DN-hTERT eukaryotic expression vector DN-hTERT-IRES2- EGFP and empty vector IRES2-EGFP were transfected into MCF7 cells using lipofectamine2000. Af-ter selection with G418, positive clones were obtained. Growth of the transfected cells was observed with inverted fluorescence microscopy. The expression levels of hTERT mRNA in the transfected cells was determined by reverse transcriptase polymerase chain reaction (RT-PCR). Telomerase activity in transfected cells was measured by TRAP-ELISA. Telomeric length was measured by FLOW-FISH. The tumorigenic potential of transfected cells was observed in nude mice. Results : The proliferation of MCF7 cells transfected with DN-hTERT-IRES2-EGFP (MCF7/DN) was inhibited. The expression levels of hTERT mRNA increased in MCF7/DN cells. Telomeric length was shorter in MCF7/DN cells than in MCF7 cells transfected with empty vector (MCF7/I). The telomerase activity measured by TRAP—ELISA in MCF7/DN cells was 2.36土0.12 and in MCF7/I it was 3.32士0.14，a difference that is statistically significant (P<0.05). The tumorigenic ratio in nude mice was 0% for MCF7/DN cells and35% for MCF7/I cells after 2 weeks. There was a significant difference between the ratios for MCF7/ DN and MCF7/I (P<0.01). Conclusion: DN-hTERT inhibits malignant phenotype and telomerase ac-tivity in MCF7 cells.